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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 631 records · Page 35
All sourcesNASA NTRSDOE OSTI
NASA NTRS1991

Navier-Stokes simulation of the supersonic combustion flowfield in a ram accelerator

A computational study of the ram accelerator, a ramjet-in-tube device for accelerating projectiles to ultrahigh velocities, is presented. The analysis is performed using a fully implicit TVD scheme that efficiently solves the Reynolds-averaged Navier-Stokes equations and the species continuity equations associated with a finite rate combustion model. Previous analyses of this concept were based on inviscid assumptions. The present results indicate that viscous effects are of primary importance; in all the cases studied, shock-induced combustion always started in the boundary layer. The effects of Mach number, mixture composition, pressure, and turbulence are investigated for various configurations. Two types of combustion processes, one stable and the other unstable, were observed depending on the inflow conditions. In the unstable case, a detonation wave is formed, which propagates upstream and unstarts the ram accelerator. In the stable case, a solution that converges to steady-state is obtained, in which the combustion wave remains stationary with respect to the ram accelerator projectile. The possibility of stabilizing the detonation wave by means of a backward facing step is also investigated. In addition to these studies, two numerical techniques were tested. These two techniques are vector extrapolation to accelerate convergence, and a diagonal formulation that eliminates the expense of inverting large block matrices that arise in chemically reacting flows.

Yungster, Shaye↗
NASA NTRS1991

Supersonic quasi-axisymmetric vortex breakdown

An extensive computational study of supersonic quasi-axisymmetric vortex breakdown in a configured circular duct is presented. The unsteady, compressible, full Navier-Stokes (NS) equations are used. The NS equations are solved for the quasi-axisymmetric flows using an implicit, upwind, flux difference splitting, finite volume scheme. The quasi-axisymmetric solutions are time accurate and are obtained by forcing the components of the flowfield vector to be equal on two axial planes, which are in close proximity of each other. The effect of Reynolds number, for laminar flows, on the evolution and persistence of vortex breakdown, is studied. Finally, the effect of swirl ration at the duct inlet is investigated.

Kandil, Osama A.↗
NASA NTRS1991

Thin-layer and full Navier-Stokes, locally-conical and three-dimensional asymmetric solutions

The unsteady, compressible thin-layer and full Navier-Stokes equations are used to numerically simulate steady and unsteady asymmetric, supersonic, locally-conical flows around a 5-deg-semiapex angle circular cone. The main computational scheme used for the present computations is the implicit, upwind, flux-difference splitting, finite-volume scheme. Comparisons of the solutions using the two sets of equations are presented for the flow asymmetry and its control. Computational studies are also presented to investigate the effects of the freestream Reynolds number and the locally-scaled Reynolds number on the flow asymmetry. These studies are carried out using the full Navier-Stokes equations. Three-dimensional, asymmetric flow solutions are also presented for a 5-deg-semiapex angle cone of unit length and a cone-cylinder configuration. The three-dimensional solutions are obtained by using the thin-layer equations and short-duration transient side-slip disturbances along with a very fine grid.

Kandil, O. A.↗
NASA NTRS1990

Mapping considerations for optimal binary correlation filters

The optimality of correlation filters is an important issue in applications of pattern recognition. Both binary phase-only filters (BPOFs) and amplitude encoded binary phase-only filters (AE BPOFs) are considered and the results of optimizing the filters for a real world object (the Space Shuttle) are studied. It is found that while only small improvements result from optimizing a BPOF, optimization of the AE BPOF is quite important in obtaining a useful correlation function. In the case of an AE BPOF, both signal-to-noise and peak-to-sidelobe measures must be studied. Computer simulation and experimental correlation results are presented.

Downie, John D.↗
NASA NTRS1991

Ignition delay of a gas mixture above a liquid fuel pool

A computational study has been made of transient heat transfer and fluid flow of an axisymmetric two-layer gas-and-liquid system heated from above by a small hot spot (e.g., a hot wire or pilot flame) located close to the liquid surface. The gas phase is unconfined above the liquid pool. The effects of varying gravity level (0.0001 to 1 gn), liquid pool height, and heater height are investigated. Thermocapillary convection induced by the nonuniform heating of the liquid surface combines with buoyancy forces to affect the heat transfer and the transport of fuel vapor toward the heat source. At reduced gravity, gas-phase conduction is comparable to the incident radiation at the liquid surface, whereas at 1 gn, buoyant convection carries the heat upward from the heat source and, therefore, the liquid is heated primarily by radiation.

Schiller, D. N.↗
NASA NTRS1991

Navier-Stokes simulation of the supersonic combustion flowfield in a ram accelerator

A computational study of the ram accelerator, a ramjet-in-tube device for accelerating projectiles to ultrahigh velocities, is presented. The analysis is performed using a fully implicit TVD scheme that efficiently solves the Reynolds-averaged Navier-Stokes equations and the species continuity equations associated with a finite rate combustion model. The present results indicate that viscous effects are of primary importance in all the cases studied, shock-induced combustion always started in the boundary layer. The effects of Mach number, mixture composition, pressure and tubulence are investigated for various configurations. Two types of combustion processes, one stable and the other unstable, were observed depending on the inflow conditions. The possibility of stabilizing the detonation wave by means of a backward facing step is also investigated. Two numerical techniques were tested: vector extrapolation, to accelerate convergence, and a diagonal formulation that eliminates the expense of inverting large block matrices which arise in chemically reacting flows.

Yungster, Shaye↗
NASA NTRS1991

Investigation of vortex development on a pitching slender body of revolution

A computational study of the unsteady flow about a pitching 3.5 caliber tangent ogive forebody is presented. The flow is simulated using the full 3D unsteady Navier-Stokes equations and a time-accurate implicit algorithm. Comparison to available experimental data for a steady 20 deg case is presented as validation. Effects of grid resolution and a comparison of solutions using full Navier-Stokes and the thin-layer approximation are included. The forebody is simulated in a 'pitch-up to 20 deg and hold' maneuver, and two different pitch axis locations are used in the study. Examination of the unsteady vorticity field for the pitch-up cases reveals the formation of strong shear layers as the body decelerates, and their roll-up into vortical structures in a process similar to that observed in airfoil dynamic stall. Pronounced vortex/surface interactions are seen which produce multiple secondary separation regions, ejection of vorticity from the surface, and embedded regions of high suction.

Stanek, M. J.↗
NASA NTRS1991

Reynolds number effects on supersonic asymmetrical flows over a cone at high angle of attack

The supersonic viscous flow over a 5-degree half-angle cone at an angle of attack of four times the cone half-angle is studied computationally using both the conical and the three-dimensional Navier-Stokes equations. The numerical solutions were obtained with an implicit, upwind-biased algorithm. Asymmetrical flowfields of the absolute-instability type are found using the conical-flow equations which agree with published results. However, the absolute instabilities of the originally symmetric flow found with the conical equations do not occur in the three-dimensional simulations, although spurious asymmetric three-dimensional flows for symmetric bodies arise if the grid resolution is insufficient in the nose region. The asymmetric flows computed with the three-dimensional equations are convective instabilities and are possible if the local Reynolds number exceeds a critical value and a fixed geometric asymmetry is imposed. A continuous range of asymmetries can be developed, depending on the size of the disturbance and the Reynolds number. As the Reynolds number is increased, the asymmetries demonstrate a bistable behavior at levels of side force consistent with those predicted using the conical equations. Below a certain critical Reynolds number, any flow asymmetries arising from a geometrical asymmetry are damped with increasing distance downstream from the geometrical asymmetry.

Thomas, J. L.↗
NASA NTRS1992

Effects Of Spatial Disturbances On Asymmetries Of Vortexes

Report describes computational study of effects of small spatial disturbances on asymmetries in vortexes downstream of slender bodies of revolution in flows at large angle of attack. Flow phenomena affect stability and maneuvering characteristics of missiles and aircraft. Understanding phenomena helps answer questions as how precisely symmetrical aerodynamic body should be, and how roughness, bumps, or other imperfections affect maneuvering characteristics.

Degani, David↗
NASA NTRS1992

Shedding Of Vortices From A Missilelike Body

Report describing computational study of subsonic flows about slender bodies of revolution focuses on effect of angle of attack on development of unsteadiness in flow - particularly unsteady shedding of vortices on leeward side. At large angles of attack, flows exhibit self-sustained oscillations.

Schiff, Lewis B.↗
NASA NTRS1991

Engine dynamic analysis with general nonlinear finite element codes

A general engine dynamic analysis as a standard design study computational tool is described for the prediction and understanding of complex engine dynamic behavior. Improved definition of engine dynamic response provides valuable information and insights leading to reduced maintenance and overhaul costs on existing engine configurations. Application of advanced engine dynamic simulation methods provides a considerable cost reduction in the development of new engine designs by eliminating some of the trial and error process done with engine hardware development.

Adams, M. L.↗
NASA NTRS1992

Thermal stratification potential in rocket engine coolant channels

The potential for rocket engine coolant channel flow stratification was computationally studied. A conjugate, 3-D, conduction/advection analysis code (SINDA/FLUINT) was used. Core fluid temperatures were predicted to vary by over 360 K across the coolant channel, at the throat section, indicating that the conventional assumption of a fully mixed fluid may be extremely inaccurate. Because of the thermal stratification of the fluid, the walls exposed to the rocket engine exhaust gases will be hotter than an assumption of full mixing would imply. In this analysis, wall temperatures were 160 K hotter in the turbulent mixing case than in the full mixing case. The discrepancy between the full mixing and turbulent mixing analyses increased with increasing heat transfer. Both analysis methods predicted identical channel resistances at the coolant inlet, but in the stratified analysis the thermal resistance was negligible. The implications are significant. Neglect of thermal stratification could lead to underpredictions in nozzle wall temperatures. Even worse, testing at subscale conditions may be inadequate for modeling conditions that would exist in a full scale engine.

Kacynski, Kenneth J.↗
NASA NTRS1992

Direct simulation of high-speed mixing layers

A computational study of a nonreacting high-speed mixing layer is performed. A higher order algorithm with sufficient grid points is used to resolve all relevant scales. In all cases, a temporal free-stream disturbance is introduced. The resulting flow is time-sampled to generate a statistical cross section of the flow properties. The studies are conducted at two convective Mach numbers, three free-stream turbulence intensities, three Reynolds numbers, and two types of initial profiles-hyperbolic tangent (tanh) and boundary layer. The boundary-layer profile leads to more realistic predictions of the transition processes. The predicted transition Reynolds number of 0.18 x 10(exp 6) compares well with experimental data. Normalized vortex spacings for the boundary-layer case are about 3.5 and compare favorably with the 1.5 to 2.5 found in experimental measurements. The tanh profile produces spacings of about 10. The growth rate of the layer is shown to be moderately affected by the initial disturbance field, but comparison with experimental data shows moderate agreement. For the boundary-layer case, it is shown that noise at the Strouhal number of 0.007 is selectively amplified and shows little Reynolds number dependence.

Mukunda, H. S.↗
NASA NTRS1992

Buoyant thermocapillary flow with nonuniform supra-heating. I - Liquid-phase behavior. II - Two-phase behavior

The present computational study of transient heat transfer and fluid flow in a circular pool of n-decane which is undergoing central radiative heating from above gives attention to the volumetric absorption of the radiation incident on the pool surface. The first part of this study notes that buoyancy influences the number and recirculation rates of the subsurface vortices by stabilizing hot subsurface fluid above the colder core fluid; this affects the liquid surface temperature profile and in turn governs the velocity profile that is due to thermocapillarity. In the second part, the effects of gas-liquid phase coupling, variable density and thermophysical properties, and vaporization are considered.

Schiller, David N.↗
NASA NTRS1992

Multibody interference at transonic Mach numbers

An experimental and computational study has been made of the forces and moments on multibody configurations at Mach numbers from 0.6 to 1.2. The interference forces and moments that occur on symmetric double and triple body configurations at zero angle of attack are examined in detail. In all cases, the normal interference force is always in a direction to pull the bodies together and the moment about the 0.5L point is always to rotate the tips of the bodies away from the center of the configuration. The axial interference force (pressure drag at zero angle of attack) increases the drag per store as additional stores are added to the configuration. The effect of angle of attack on the interference is relatively small.

Chapman, Gary T.↗
NASA NTRS1992

Role of shock dynamics in transonic flutter

A computational study of the influence of shock motion on flutter and divergence in transonic flow is presented. The numerical scheme models the entire fluid-structure system as a single continuum dynamics problem, by using a mixed Eulerian-Lagrangian formulation. No assumptions of small displacements are made, but the effect of viscosity is neglected. The results from this study indicate that the shock dynamics gives rise to limit cycles and highly nonlinear aeroelastic phenomena, such as weak divergence and flutter-divergence interactions. Although the shocks typically are destabilizing at the linear flutter boundary, they often have a strongly stabilizing effect for moderate-amplitude motions. The shocks are thus capable of quenching an emerging bending-torsion flutter motion and turning it into limit cycle flutter. The usefulness of classical flutter and divergence boundary diagrams is severely limited in transonic flow, because much of the global dynamic stability information is lost in such a presentation.

Bendiksen, Oddvar O.↗
NASA NTRS1992

A large-field laser holographic focusing schilieren system

A large-field laser holographic focusing schlieren system for high-speed flow visualization has been built and evaluated. This system is based on a recently improved large-field focusing schlieren technique and is combined with laser holography methods to record three-dimensional flows. A coordinated experimental and computational study of supersonic flows over wedge, cone, and sphere geometries was conducted to evaluate the capabilities and limitations of the system. The ability of the system to focus on planes normal to its optical axis is demonstrated. The sharpness of focus of the present system was found to be limited. Issues regarding obtaining quantitative measurements of the density gradient are discussed.

Doggett, Glen P.↗
NASA NTRS1993

Computations Of Flows In Flapper Valves

Three reports describe computational studies of flows of liquid oxygen and liquid hydrogen in flapper valves located at quick-disconnect interface between main propellant feedlines of Space Shuttle orbiter and Space Shuttle external tank.

Kandula, Mastanaiah↗